1. Field of the Invention
The present invention relates to an AM (Amplitude-Modulated) transmitter and to a modulation method using the same.
2. Description of the Related Art
A conventional AM transmitter is disclosed in, for example, Unexamined Japanese Utility Model Application KOKAI Publication No. S63-72942 (page 3-4, FIG. 1) which is configured to modulate a transmitting signal to be outputted and to detect a nonlinear distortion by signal detection and to make variable a level of an input signal to a power amplifier using a gain controller for feedback.
The disclosed AM transmitter, as shown in FIG. 3, includes a directional coupler 51, a modulator 52, a detector 53, a detected output amplifier 54, an automatic gain controller 55, and a power amplifier 56.
The directional coupler 51 is configured to make a carrier wave component fed by the power amplifier 56 branch and to supply the branched carrier wave to the modulator 52. The modulator 52 is configured to amplitude-modulate, using a modulating signal, the carrier wave component supplied from the modulator 52. The detector 53 is configured to detect the modulated signal which is amplitude-modulated by the modulator 52.
The detected output amplifier 54 is configured to amplify a detecting signal resulting from the detection by the detector 53 up to a specified level and to feed the amplified detecting signal to the automatic gain controller 55. The automatic gain controller 55 is configured to control a level of a carrier wave of an input signal to be inputted to the power amplifier 56 according to an output from the detected output amplifier 54.
However, in the conventional AM transmitter, as shown in FIG. 3, the automatic gain controller 55 to perform modulation is mounted in the front stage of the power amplifier 56. As a result, the modulation distortion of a transmitted output depends greatly on linearity of the power amplifier 56.
For example, as shown in FIG. 4, in the AM transmitter having an output power of 1 W occurring at a non-modulation time, at a time of 100% modulation, an output power of 4 W is required. That is, it is necessary that input-output power characteristics provided by the power amplifier 56 exhibit linearity till its output power reaches 4 W.
However, in reality, since the power amplifier 56 has a nonlinear distortion, in order to obtain an output power of 4 W, the power amplifier 56 that can provide an output power of, for example, 8 W is required.
That is, in order to improve a modulation distortion appearing when deep modulation is performed, the power amplifier 56 providing large peak power is required. If the power amplifier 56 providing such large peak power is applied to the AM transmitter, its miniaturization is made difficult and, in a small-sized AM transmitter in particular, the use of such the power amplifier causes disadvantages.
Moreover, in the AM transmitter disclosed in the Unexamined Japanese Utility Model Application KOKAI Publication No. S63-72942, by modulating a carrier wave component of a transmitting signal and by performing detection, a difference among nonlinear components in the power amplifier 56 is calculated and by controlling a level of a carrier wave to be inputted to the power amplifier 56, the nonlinear distortion is compensated for.
The modulator 52 modulates a carrier wave made to branch by the directional coupler 51, and furthermore, the automatic gain controller 55 modulates a carrier wave of an input signal. This causes complicated configurations of the AM transmitter.